Process for the purification of flue gas

ABSTRACT

A process for the purification of flue gas, comprising the step of contacting flue gas with a carbonaceous material comprising a solid carbonaceous residue of synthetic rutile production from titaniferous ores.

The invention is directed to a process for the purification of flue gas,wherein flue gas is contacted with a carbonaceous material.

Various industrial processes provide vast amounts of flue gas streamscontaining environmentally hazardous substances, such as fly ash, acidgases Nox, dioxins, furans, and heavy metal compounds. Examples of suchindustries are waste incinerators burning various feeds (municipalwaste, clinical waste, hazardous waste), the metallurgical industry, themetal recovery industry, power plants, cement plants and the like.

In order to reduce the emission of hazardous substances, many industriesare obliged to clean up their flue gases before ventilation in theenvironment. Depending on the nature of the pollutant, varioustechniques have been developed to clean up the flue gas. For example,fly ash can be removed with electrostatic precipitators (ESP), fabricfilters (FF) or wet scrubbers. Acid gases are mostly bound to alkalinecompounds, either in a (semi-) dry system with spray dryer adsorbers(SDA), or in wet systems using scrubbers. Many flue gas cleaninginstallations have been built containing these basic components.

For the removal of dioxins, furans and mercury compounds from flue gasoften additional measures have to be taken in order to comply with thecurrent emission limits. Mostly, the flue gases are brought into contactwith an adsorbent to bind these compounds.

A well-known method to remove dioxins, furans and mercury compounds isto inject a powdered adsorbent in the ducts of a flue gas cleaningsystem, after which hazardous compounds adsorb onto the adsorbent. Insubsequent parts of the installation the spent adsorbent is removed fromthe flue gas in a particle collection system. The collection of theadsorbent is often performed in existing ESP, FF or wet scrubbers, whichmakes this technology especially suited for existing flue gas cleaninginstallations. A vast amount of patents have been granted describingvarious flue gas cleaning installation modifications applying powderedadsorbents for flue gas cleaning.

The conditions under which adsorbents are applied depend to a largeextent on the nature of the industrial process generating the flue gasesand on the modification of the flue gas cleaning installation.

In general, flue gas consists of fly ash and various gases and volatilecompounds, such as nitrogen, oxygen, carbon dioxide, nitrogen oxides,water, carbon monoxide, sulphur dioxide, and various acid gases. Theprecise composition of the flue gas is determined by the nature of theprocess generating the flue gas and can vary significantly in time. Asuitable adsorbent must be able to withstand these variations of theflue gas composition.

The maximum temperature at which powdered adsorbents can be used ispartly determined by the maximum operating temperature of the particlecollection system. For ESP and FF the maximum operating temperatures aretypically 450° C., respectively 300° C. In wet scrubbers the maximumoperating temperature is always below 100° C. The maximum applicationtemperature is preferably kept below 250° C. to prevent the formation ofadditional dioxins due to the so-called de novo synthesis route.

Various adsorbents are used for the cleanup of flue gas. Commonlyreported adsorbents for this application are activated carbon andactivated lignite cokes.

The adsorption capacities of activated carbon and activated lignitecokes for dioxins and furans can be extremely diverse, depending amongstothers on the nature of the raw material and on the method ofproduction. Usually, the carbon types used in flue gas cleaning areproduced from raw materials like peat, coal, or lignite, produced bystream activation processes. Alternatively, carbon or activated carbonwaste. The PAC types based on reactivated carbon or on activated carbonwaste generally have a varying quality die to the varying quality of theraw material.

The main properties determining the quality of activated carbon for fluegas cleaning are the adsorption properties and the ignition properties.

The adsorption properties are mainly determined by the pore structureand by the particle size distribution of the powdered activated carbon.The pore structure of the carbon is defined by the nature of the rawmaterial and by the process conditions during activation. A suitableactivated carbon preferably contains a high micropore volume for a highadsorption capacity, next to a high mesopore volume for a rapidtransport of the adsorbates to the adsorbing pores. The particle sizedistribution is primarily determined by the quality of the millingequipment.

When applying powdered carbon under oxidising conditions at elevatedtemperatures as in flue gas, the possibility of ignition of the carbonhas to be taken into account. Typically, the temperature in the ESP orFF of the flue gas cleaning system ranges from 100 to 200° C. In somecases the temperatures are even higher.

Ignition of carbon adsorbents is usually first observed in the dustcollection sections of an ESP or a FF, since on these spots warmed upcarbon can accumulate. Under sufficiently severe conditions in principleall carbon adsorbents can eventually ignite, resulting in undesiredexcessive temperature increases. Changes in the design of theinstallation can reduce the ignition hazard. Choosing the proper carbonadsorbent can reduce the ignition hazard as well.

In general the ignition properties of an activated carbon or othermaterial used in flue gas cleaning systems are determined using astandard ignition test. Such tests are defined in the Recommendationsfor the transport of dangerous goods, 9th revised edition, UnitedNations, 1995, parts 14.5.5 and 33.3.1.3.3.

Next to the adsorption and ignition properties, secondary propertiessuch as material availability and production costs also determine thesuitability of an adsorbent for flue gas cleaning.

It is an object of the present invention to provide an alternative tothe presently used powdered activated carbon, whereby the ignitioncharacteristics of the material are improved.

It is also an object of the present invention to provide a carbonaceousmaterial suitable for flue gas purification, wherein the material has animproved balance of properties in relation to adsorption characteristicsand ignition behaviour.

The present invention is based on the surprising discovery of a materialthat meets these objects, when applied in flue gas purification.Surprisingly, a new carbonaceous adsorbent material was found having apore structure that is likely superior to that of activated carbonscommonly used for flue gas cleaning. The new material is produced as aby-product in the synthetic rutile production industry and has excellentignition properties. These combined properties make this new adsorbentespecially suitable for flue gas cleaning.

The solid carbonaceous material is produced as waste product during theproduction of synthetic rutile from titaniferous ores (ilmenite,leucoxene, or slag). During the production of synthetic rutile, carbonis used for the chemical reduction of iron within the titaniferousminerals, possibly in combination with chlorine. The reduced iron issubsequently removed from the minerals to obtain synthetic rutile. (SeeUllmann's Encyclopedia of Industrial Chemistry, Sixth Ed., 199Electronic Release, Wiley-VHC, Weinheim (DE) on Titanium Dioxide, §2.1.2.2 Synthetic Raw Materials).

After recovery of the synthetic rutile from the solid material acarbonaceous waste product remains, which has been found to have a porestructure corresponding to the pore structure of activated carbons thatare suitable for adsorption of contaminants such as dioxins, furans andmercury compounds from flue gas. If necessary the material can bepurified, sieved and/or ground to obtain the optimal properties. More inparticular, the particle size may need to be regulated, depending on thetype of system used. Generally the material is modified to have aparticle size between 1 and 100 μm.

The carbonaceous material can be used in the same manner as thepresently used powdered carbons, by injecting them at a suitablelocation in the flue gas. This can be done in the dry form, as wettedmaterial and/or in combination with alkaline materials, such as lime toremove acidic substances from the flue gas. After the material hasadsorbed the contaminants, it is again removed from the gas, for exampleby ESP or FF.

The flue gas has generally been subjected to some treatment prior to theintroduction of the carbonaceous material, such as cooling to recoversome heat from it, removal of fly ash, and the like. More in particular,the flue gas may be cooled to a temperature between 0 and 500° C.,before contacting it with the solid carbonaceuos material.

The invention is now elucidated on the basis of the following examples,which are not intended to limit the scope of the invention.

EXAMPLE 1

The pore structure of activated carbons is generally divided into threemajor size ranges: micropores (pore radius<1 nm), mesopores (1 nm<poreradius<25 nm), and macropores (pore radius>25 nm). The respective porevolumes are generally derived from adsorption experiments with standardadsorbates (micropores and mesopores), or from mercury porosimetry(macropores and larger mesopores). With activated carbons used for thepurification of gases, the micropores and the mesopores (adsorbingpores) are generally used for adsorption of adsorbates, whereas themacropores and larger mesopores (transporting pores) are used fortransport of adsorbates from the surroundings to the adsorbing pores. Asuitable activated carbon for flue gas cleaning contains both adsorbingpores and transporting pores in sufficient amounts, to provide optimumadsorption capacity and fast adsorption kinetics. For powdered activatedcarbon types the macropores have largely disappeared due to the millingprocess.

A commonly accepted analytical parameter for activated carbon is theso-called iodine number. The iodine number is the amount of iodineadsorbed onto activated carbon (in mg iodine/g carbon) in equilibriumwith a 0.02 N iodine solution The test method has been describedextensively in ASTM D 4607-86. The iodine number of activated carbon isrelated to its micropore volume. An alternative parameter indicating themicropore volume of activated carbon is the equilibrium butaneadsorption capacity when the carbon is brought into contact with dry aircontaining 0.24 vol % butane. The iodine number is thus related to thevolume of the adsorbing pores.

A parameter indicating the combined pore volume of larger mesopores andsmall macropores is the molasses number. The molasses number is definedas the number of milligrams activated carbon required to achieve thesame decolorizing effect as 350 mg of a standard carbon, determinedusing a standard molasses solution by a standard procedure. Due to thelarge size of the molasses molecules only large pores can be entered,therefore, the molasses number is an indication for the volume of thetransporting pores. In this case, the molasses number decreases as thetransporting pore volume increases.

Table 1 contains typical iodine numbers and the molasses numbers ofseveral activated carbon types that are commonly used for flue gascleaning, as well as those of the carbonaceous residue produced in thesynthetic rutile production process. Based on these values, theadsorption properties and adsorption kinetics of the carbonaceousresidue are more favorable for flue gas cleaning compared to thecurrently applied carbon types, because both adsorption and transportpore volumes are higher.

TABLE 1 Typical Iodine numbers and Molasses numbers of various activatedcarbons used for flue gas cleaning, and of the carbonaceous residuematerial. Iodine number Molasses number Carbon type [mg/g] [a.u.] DarcoFGD 550 400 NORIT GL 50 700 475 NORIT SA Super 1050 200 Carbonaceousresidue 1200 150

EXAMPLE 2

The auto-ignition hazard of stationary activated carbon layers can beassessed by determining the so-called critical ignition temperature(CIT). The test method for determining the CIT of powdered activatedcarbon is similar to a test mentioned in the “Recommendations on thetransport of dangerous goods, issued by the United Nations”, section14.5.5 (ST/SG/AC.10/1/Rev.9). This test is designed to establish whetheror not self-heating substances can be transported in bulk. In theUN-test it is determined if a carbon sample in a 1 liter cube (10×10×10cm) auto-ignites at a fixed temperature of 140±2° C. An elaboratedescription of this test can be found in the above-mentioned manual.

The CIT test method is in principal identical to the UN test method,only the temperature at which the sample is tested is made variable.Depending on the outcome of the first test at a pre-selectedtemperature, a new test temperature is chosen and a fresh carbon sampleis tested. This is repeated until the highest temperature at which noignition took place and the lowest temperature at which ignition didtake place are about 10° C. apart. The CIT is defined as the average ofthese temperatures.

Table 2 contains the CIT values of several activated carbon types thatare commonly used for flue gas cleaning, as well as that of thecarbonaceous residue produced in the synthetic rutile productionprocess. The data in Table 2 clearly indicate that the CIT of thecarbonaceous residue is significantly higher than that of the regularflue gas cleaning carbon types.

TABLE 2 Typical critical ignition temperatures (CIT) and averageparticle size of various activated carbons used for flue gas cleaning,and of the carbonaceous residue material. Particle size (d₅₀) CIT Carbontype [μm] [° C.] Darco FGD 14 240 NORIT GL 50 17 250 NORIT SA Super 7270 Carbonaceous residue 33 330

1. Process for the purification of flue gas, comprising the step ofcontacting flue gas with a carbonaceous material, comprising a solidcarbonaceous residue of synthetic rutile production from production fromtitaniferous ores.
 2. Process according to claim 1, further comprisinginjecting said carbonaceous material into the flue gas to be purified.3. Process according to claim 2, further comprising removing saidcarbonaceous from the flue gas after sufficient contact time foradsorbing contaminants from the flue gas.
 4. Process according to claim3, further comprising: cooling flue gas to a temperature between 0 and500° C., before contacting it with said solid carbonaceous material;removing dioxins, furans and mercury compounds from the flue gas;contacting said solid carbonaceous material with the flue gas in a drystate, wet state and/or in combination with lime; and sieving,purifying, and/or grinding said carbonaceous material prior to use. 5.The process of claim 4, wherein said flue gas originates from wasteincinerators, metallurgical facilities, metal recovery facilities, powerplants or cement plants.
 6. Process according to claim 2, furthercomprising: cooling the flue gas to a temperature between 0 and 500° C.,before contacting it with said solid carbonaceous material; removingdioxins, furans and mercury compounds from the flue gas; contacting saidsolid carbonaceous material with the flue gas in a dry state, wet stateand/or in combination with lime; and sieving, purifying, and/or grindingsaid carbonaceous material prior to use.
 7. The process of claim 6,wherein said flue gas originates from waste incinerators, metallurgicalfacilities, metal recovery facilities, power plants or cement plants. 8.Process according to claim 1, further comprising cooling the flue to atemperature between 0 and 500° C., before contacting it with the saidsolid carbonaceous material.
 9. Process according to claim 1, whereinsaid flue gas originates from waste incinerators, the metallurgicalfacilities, metal recovery facilities, power plants or cement plants.10. Process according to claim 1, further comprising removing dioxins,furans and mercury compounds from the flue gas.
 11. Process according toclaim 1, further comprising contacting said solid carbonaceous materialwith the flue gas in dry state, wet state and/or in combination withlime.
 12. Process according to claim 1, further comprising sieving,purifying, and/or grinding said carbonaceous material prior to use.